Y. June Xu
Louisiana is naturally blessed with an abundance of bayous, rivers, lakes and aquifers, which provide Louisiana’s citizens with fishing, hunting, boating and recreational opportunities and contribute to the state’s wealth and economic growth. While the state has more surface water available (84 percent) than any other state in the United States, rapid urbanization and intensive agricultural and forestry practices have increased the potential for deterioration of the quality of the state’s surface waters.
Watersheds are increasingly becoming the primary planning unit for natural resource management. Louisiana uses a set of 475 sub-segment watersheds in 12 river basins as a spatial framework for its surface water quality assessment. The Louisiana Department of Environmental Quality (LDEQ) maintains a statewide water quality database that compiles data collected from more than 600 locations across the state’s 475 sub-segment watersheds. The state agency collects and analyzes chemical, physical and biolog-ical properties of water, sediment and tissue samples from Louisiana’s bayous, streams, rivers and lakes.
In addition to the LDEQ’s surface water surveillance, the U.S. Geological Survey (USGS) collects water quality samples across the state and maintains a statewide stream-flow monitoring network. This network ensures continuous measurements in hundreds of bayous, streams and rivers. Using the datasets provided by these agencies, LSU AgCenter researchers are conducting several studies on surface water quality at the watershed scale.
Variation among landforms
LSU AgCenter researchers are synthesizing water quality data collected from Louisiana’s bayous, streams and rivers. The goal of the project is to assess long-term water quality changes across the five major landforms in the state – coastal marsh to the coastal plain, Mississippi alluvial valley, upland terrace and upland hills. The research will provide insights into the interrelationships between hydrological conditions, land use and the water quality of inland streams, wetlands and coastal estuaries in Louisiana. The knowledge gained from the research will contribute to developing site-specific water quality standards and facilitating assessment of the effective-ness of farm management practices in water quality protection.
Preliminary results show that Louisiana’s coastal streams have considerably higher nutrient but lower dissolved oxygen levels during much of the year, compared to those in the upland drainage basins. Over the past 24 years, average dissolved oxygen levels in all studied drainage basins remained relatively consistent. The relationship between dissolved oxygen and water temperature in the coastal streams was closer than that of the upland streams. These results accentuate the need for site-specific water quality standards.
Climate, land use and Lake Pontchartrain
Freshwater and sediment from upland tributaries are critical to the development of the diverse wetland, marsh and aquatic ecosystems in the Lake Pontchartrain Basin. There is concern that the combination of climate and land use changes may dramatically affect the freshwater input and that the changes, therefore, may pose a threat to the stability of eco-systems in southeast Louisiana. LSU AgCenter researchers are conducting a study on long-term changes in discharge and sediment from the Amite, Tickfaw and Tangipahoa river watersheds (Figure 1). The research focuses on two questions:
The study shows that, on average, the three watersheds delivered 5 cubic kilometers of freshwater each year into Lake Pontchartrain. The discharge in these watersheds was highest from January to May and lowest from July to October (Figure 2), indicating a potential threat of high nutrient runoff into Lake Pontchartrain during the springtime.
Nitrogen removal and the Atchafalaya Basin
Nitrogen enrichment from the upper Mississippi River basin has been cited as the major cause for the hypoxia (lack of oxygen) in the Gulf of Mexico. The hypoxia, also called the “dead zone,” threatens Louisiana’s fishery industry among other problems. Although freshwater diversion from the lower Mississippi River into the region’s wetlands has been consid-ered an effective method of reducing nitrogen loads, it is largely uncertain how much nitrogen actually can be retained from the overflowing waters. Generally, there is a knowledge gap in what tools are available for accurate assessment of nitrogen inflow, outflow and removal potential for those diverse coastal floodplain systems.
A study addressing this question for the Atchafalaya River basin is being conducted by LSU AgCenter researchers. Long-term nitrogen inflow at Simmesport and outflows at Wax Lake Outlets and Morgan City (Figure 3) are being analyzed. Nitrogen input and output budgets will be established to assess total nitrogen mass removal rates of the Atchafalaya River basin under different hydrologic regimes.
Results from this initial study show that annual average inflow of the Atchafalaya River basin was about 10 cubic kilometers higher than its average outflow. Monthly average inflow of the Atchafalaya was higher than its monthly average outflow from December to June, but lower than its monthly average outflow from August to October (Figure 4).
Over the past 68 years, the river’s hydraulic gradient between Simmesport and Morgan City decreased steadily at an average rate of 4.8 centimeters per year. This means the river is becoming flatter, probably due to sediment build-up. We are not sure what the consequences of this situation may be. These results suggest that the nitrogen removal capacity of the Atchafalaya River basin is likely dependent on the inundation extent of the basin area and that establishing a spatial relationship between the river stage and inundation extent can be useful for the basin’s management.
Y. Jun Xu, Assistant Professor, School of Renewable Natural Resources, LSU AgCenter, Baton Rouge, La.
(This article appeared in the spring 2004 issue of Louisiana Agriculture.)